Production of oxalate coatings on iron and steel



United States Patent PRODUCTION OF OXALATE COATINGS ON IRON AND STEEL Paolo de Cerma, Milan, Italy, assignor to Montecatini, Societa Generale per IIndustria Mineraria e Chimica, a corporation of Italy No Drawing. Application April 5, 1956 Serial No. 576,238

Claims priority, application Italy April 8, 1955 4 Claims. (Cl. 148-6.14)

My invention relates to a process for forming oxalate coatings on metal surfaces, in particular iron and steel surfaces.

Oxalate coatings are used for the protection of iron metals and constitute an excellent basis for paint.

The production of oxalate coatings on iron and steel by using baths based on oxalic acid and sodium chlorate is known. However, such coatings, if used as backing for paint, show a notable deficiency. After the coating is formed, the metal surface is powdery so that sometimes the desired anchoring of the paint is not achieved or deteriorates appreciably.

It is an object of my invention to eliminate such shortcomings.

To this end, and in accordance with my invention, the sodium chlorate in the process of forming oxalate coatings is replaced by another oxidizing agent, particularly sodium nitrite. I have found that in this manner a perfectly smooth and powder-free oxalate coating is obtained. To achieve this result the concentration of the sodium nitrite in the oxalic-acid bath must be maintained within the limits of 0.2 to 4 grams per litre, the optimum concentration being 1 to 2 g./l. The applicable concentration for the oxalic acid in the bath ranges from 3 g. to 50 g./l.

The nitrite, used in form of an aqueous solution, may be fed batchwise; but the results obtained are better and above all more constant if the feeding is continuous by dripping the sodium nitrite solution into the bath. It has further been found that the starting of the bath is easier if a ferrous salt is added initially in such an amount that the bath contains from 0.1 to 1 g./l. of ferrous ions. It is not necessary to subsequently add further amounts of this salt since during operation of the bath small amounts of ferrous ions go continuously into solution. The bath according to the invention can be employed either by dipping or spraying.

The following examples will illustrate, but are not meant to limit, the invention:

Example 1 Soft-steel sheets are treated by dipping or spraying at room temperature for 3-4 minutes with oxalic acid solution containing 15 g./l. sodium chlorate. A ferrous oxalate coating is obtained, which is continuous and compact but covered by an incoherent powder which renders the surface diflicult to paint. The result is not improved if the concentration of sodium chlorate is increased or decreased.

Example 2 Soft-steel sheets are treated with a solution containing 15 g./l. oxalic acid and 0.1 g./l. sodium nitrite, under the same conditions of Example 1. An incoherent and discontinuous coating is formed.

Example 3 Soft-steel sheets are treated under the same conditions 2,863,791 Patented Dec. 9, 1958 Example 4 Soft-steel sheets are treated under the same conditions as in Example 1 with a solution contatining 15 g./l. oxalic acid and 1.5 g./l. sodium nitrite. The coating obtained is continuous, compact and free from incoherent powder.

Example 5 Soft-steel sheets are treated under the same conditions as in Example 1 but with a solution containing 15 g./l. oxalic acid and 4 g./l. sodium nitrite. A continuous and compact coating showing slight formation of incoherent powder is obtained.

Example 6 Soft-steel sheets are treated under the conditions of Example 1 but with a solution containing 15 g./l. oxalic acid and 6 g./l. sodium nitrite. A coating is obtained which is continuous and compact but covered with an incoherent powder as obtained in Example 1.

As stated above, the concentration of the oxalic acid in the bath, ranges from 3 g. to 50 g./l. however it is preferably maintained at 15 g./l., as described in the examples.

It appears from the examples that for obtaining best results and completely eliminating the defect of the known method it is necessary to operate in the presence of sodium nitrite within the concentration limits according to the invention. In fact, from Example 4 it is evident that the concentration of sodium nitrite is a factor capable of decidedly modifying the quality of the oxalate coating.

I claim:

1. The process of forming oxalate coatings on iron and steel which comprises applying to the body to be coated an aqueous solution of oxalic acid and sodium nitrite, and maintaining the nitrite concentration between 0.2 and 4 grams per litre.

2. The process of forming oxalate coatings on iron and steel which comprises applying to the body to be coated an aqueous solution of oxalic acid and sodium nitrite, and maintaining the nitrite concentration between 1 and 2 grams per litre.

3. The process of forming oxalate coatings on iron and steel which comprises passing the bodies to be coated through a bath of oxalic acid and sodium nitrite, and continuously adding the nitrite to the bath to maintain a nitrite concentration between 0.2 and 4 grams per litre.

4. The process of forming oxalate coatings on iron and steel which comprises applying to the body to be coated an aqueous solution of 3 to 50 g. per liter of oxalic acid and sodium nitrite, adding initially to the solution an amount of ferrous salt corresponding. to an initial ferrous ion concentration of 0.1 to 1 gram per litre, and maintaining the sodium nitrite concentration between 0.2 and 4 grams per litre.

Weisberg et a1 Feb. 21, 1939 Tanner Feb. 17, 1942 

1. THE PROCESS OF FORMING OXALATE COATINGS ON IRON AND STEEL WHICH COMPRISES APPLYING TO THE BODY TO BE COATED AN AQUEOUS SOLUTION OF OXALIC ACID AND SODIUM NITRITE, AND MAINTAINING THE NITRITE CONCENTRATION BETWEEN 0.2 AND 4 GRAMS PER LITRE. 